The invention relates to a spheroidal cast alloy for cast iron products with great mechanical strength, high wear resistance and at the same time a high degree of ductility, comprising as non-iron constituents 2.5 to 3.8% by weight C, 2.4 to 3.4% by weight Si, 0.02 to 0.08% by weight P, 0.02 to 0.06% by weight Mg, 0.01 to 0.05% by weight Cr, 0.002 to 0.02% by weight Al, 0.0005 to 0.015% by weight S, 0.0002 to 0.002% by weight B and the conventional impurities.
In motor vehicle construction, cast iron alloys are used for producing cast parts that must have high wear resistance, for example brake disks, which during the braking operation have to convert the kinetic energy of the vehicle into thermal energy. The brake disks can in this case reach temperatures of up to about 850° C. During the braking operation, not only the brake linings but also the brake disks are worn. Brake disks have irregular wear and often have to be replaced while still under warranty, involving high costs for the automobile manufacturer. In order that the wear on the surface of the brake disk takes place as evenly as possible, high demands are made of the crystalline structure and the homogeneity of the structure. The homogeneity can be improved by a suitable casting process.
GB 832 666 discloses a cast iron alloy comprising as non-iron constituents 1.0 to 2.5% by weight C, 1.5 to 3.2% by weight Si, less than 1.15% by weight Mn, less than 0.5% by weight S and 0.001 to 0.05% by weight B. After casting, the graphite component takes on the compact form. Because the alloy does not contain any Mg there is no spheroidal graphite or vermicular graphite present, but rather a graphite formation that resembles temper carbon nodes of malleable cast iron predominates. The alloy contains 5 to 10% carbides in a predominantly pearlitic matrix, which has the consequence that the elongation at rupture becomes relatively low. In order to limit the formation of lamellar graphite, and consequently improve the modulus of elasticity, tellurium and bismuth are admixed as alloying elements. Higher elongation at rupture values are achieved by a subsequent heat treatment.
US 2004/0112479-A1 discloses a further cast iron alloy, which preferably contains 3.7% by weight C, 2.5% by weight Si, 1.85% by weight Ni, 0.85% by weight Cu and 0.05% by weight Mo. This material is distinguished by an elongation of 20 to 16% with a tensile strength of 500 to 900 MPa and by a Brinell hardness of 180 to 290 HB. These properties are achieved after a time-consuming heat treatment, which comprises the following successive steps: 10 to 360 minutes of austenitizing at temperatures between 750 and 790° C., rapid cooling in a salt bath at a temperature between 300 and 400° C., 1 to 3 hours of austempering at temperatures between 300 and 400° C. and cooling to room temperature. After this treatment, the material has a structure with an austenitic and ferritic microstructure. The material is distinguished by easier machinability than a cast iron that has been subjected to a conventional type of austempering.
On the basis of this prior art, the object of the invention is to provide a cast iron alloy which is produced from elements that are as inexpensive as possible, the cast parts having the highest or greatest possible heat resistance and strength, in particular wear resistance, and at the same time a very high degree of ductility, without an additional heat treatment.